Tetrahydrobiopterin (BH4) is required in the tyrosine hydroxylase (TYH) catalyzed synthesis of 3,4-dihydroxyphenylalanine, precursor to the catecholamines, dopamine (DA) and norepinephrine (NE) which function as neurotransmitters in the peripheral and central nervous systems (CNS). BH4 is strongly implicated in many aspects of TYH regulation. BH4 is also synthesized in the brain from GTP; and BH4 levels in brain are responsibe to d-amphetamine treatment in vivo. The proposed work addresses the enzymology, kinetics, and mechanisms related to synthesis and utilization of reduced pterins in mammalian brain. Regulation of central dopaminergic function is emphasized. TYH will be purified from bovine striate cortex and be subjected to rigorous, extensive kinetic study including kinetic isotope effects, substrate specificity and product and dead-end inhibition patterns. Three reduced pterin cofactors will be compared. The highly characterized "native" TYH will provide a background to elucidate the mechanisms underlying TYH activations which have been reported. GTP-cyclohydrolase (GTP-CH), the enzyme catalyzing the conversion of GTP to 7,8-dihydroenopterin in triphosphate (H2NPt-P3) represents the initial and apparently slowest step in pterin biosynthesis. The enzyme will be studied as a likely locus for regulation of CNS pterin synthesis and pterin levels.